The present invention relates to a seismic cable for positioning on the sea floor comprising two or more seismic sensor units adapted to detect vibrations in the sea floor and being separated by chosen lengths of cable.
Marine seismic studies are usually performed with seismic cables being towed by a vessel at the sea surface, the seismic cables comprising a number of hydrophones recording pressure waves being reflected from the geological formations beneath the sea floor, the pressure waves being generated by seismic sources such as air guns. It is, however, known that certain geological formations may convert pressure waves into shear waves that are not transmitted through water and thus are not registered by the hydrophone cables. Thus additional information are obtained by placing sensors on the sea floor being capable of registering these shear waves.
A number of different systems have been suggested to measure these waves, e.g. in U.S. Pat. Nos. 5,265,066 and 5,747,754. In these publications cables are described comprising geophones being placed on the sea floor to detect vibrations. A problem being addressed in the publications is how to obtain sufficient acoustic contact against the sea floor. In the first these is solved by making the geophone units heavy, which limits the frequencies that may be received.
In U.S. Pat. No. 5,747,754 the problem with the coupling to the ground is solved by enclosing vibrators in the geophone units. By letting the sensor units vibrate they will dig themselves down into the sea floor, and thus improve the contact. It is also said that the geophones should have a specific weight corresponding to the material at the sea floor. This is a prejudice being common in the art.
In 5,365,491 a solution is described in which one or more geophones are positioned outside the cable being enclosed in sand bags in order to improve the contact on a hard surface. This gives, however a distributed weight over the surface and thus a low weight per length unit and thus a bad acoustic coupling to the surface. This solution will therefore, if it contrary to the intent of the patent is used on a sandy surface, not give any better contact with the surface as the sand in the bags will behave in the same way as the sand outside, and the real effect will be the same as if the bags were not there and the sensor was positioned in the sand on the bottom. Thus it is not suitable to solve the same problem as the present invention.
The solution comprising sensors positioned outside the cable is well known, and has proved not to give optimal measurements, partially because their positions relative to the cable when landing on the sea floor affects the measurements. The primary reason was that they should lie relatively independent of the cable, but practice has shown that they normally land in contact with the cable on the sea floor and are thus affected e.g. by vibrations propagating along the cable. Because of operational conditions it has also become usual to tape these outer sensors to the cable, so that they, when laid on the sea floor, come underneath, beside or over the cable, which in a large degree affects the measurements. In this case also the sensors are given a specific weight corresponding to the environment.
It is an object of this invention to provide a compact and robust seismic cable in which the weight is distributed over a small area, so as to obtain good acoustic coupling to the sea floor when this consists of relatively loose materials such as sand and sediments.
It is thus an object of the present invention to provide a robust seismic cable as defined above, having well defined contact points against the sea floor. This is obtained with a seismic cable being characterized as stated in claim 1.
The seismic cable according to the invention has a cable between the sensor units having substantially smaller physical dimensions than the sensor units, which is advantageous, both regarding the influence for sea currents during deployment and positioning on the sea bed and after being positioned there. Also, the cable will demand less room when stored on a drum.
The cable may also comprise suitable, per se known, materials or solutions for damping vibrations along the cable. This may be done with suitable means for attenuate vibrations or by letting them be attenuated through the cable to the sea floor.